gdavis: Yes, that does help. It also shows that there is a specific parameter space in which Ic = BIb, that is, not in saturation (and I would assume not in cutoff). So to keep it working as a current source one has to control Ib so that it remains in the "active" state as you described. Very interesting. However, I still have a problem with the variability of beta from unit to unit and I suspect that this is why we don't find such a technique used in professional grade music equipment.

When I wrote about internal impedance changes, I was referring to the current source circuit as a whole. I do see that the current source itself is the transistor so yeah, the impedance changes are external to it._________________FPGA, dsPIC and Fatman Synth Stuff

Time flies like a banana.Fruit flies when you're having fun.BTW, Do these genes make my ass look fat?corruptio optimi pessima

Yes, you definitely have to control Ib to keep it in the desired operating region, that is what biasing is all about and an important part of any transistor circuit design.

Starting from 0 CV, it will be in cuttoff and the base voltage has to get above the cuttoff voltage before it will enter the active region and start responding. As you continue up you have to get through the "knee" before it starts responding closer to linearly. Taking advantage of the knee is how some exponential converters work. The OP's circuit will probably fail before it ever reaches saturation since there isn't really any resistance to the collector (JovianPyx noticed this as well).

So ya, this is the simplest and probably least effective implementation. There are definitely improvements to be made.

First thing I would suggest is a resistor in series with the collector and LED. Choosing an appropriate value will cause the transistor to reach saturation before a safe current is exceeded.

A resistor between the emitter and ground will help stabilize the effect of varying Hfe a bit.

Carefully choosing the values for all the resistors will allow you to control the overall gain and result in a more well behaved circuit, though still not to the level of an opamp based design, but it's a simple circuit that's easy to implement.

And now he's demonstrating a resistor as a current source! Of course it can work, but puts a heavier demand on the CV driver. But again, depending on the application, just might be good enough._________________My synth build blog: http://gndsynth.blogspot.com/

Ah, thank you - I had wondered about that "knee" and whether that was what was at the root of expo.

I am confused though, I don't see how a resistor is a current source. You mentioned that the OP used a resistor current source - I don't understand where the current regulation comes into that... is the term "current source" that loose?

Wiki defines current sourse thusly: A current source is an electronic circuit that delivers or absorbs an electric current which is independent of the voltage across it.

The diode in the circuit is nonlinear, so if the resistor is too large, the diode will not switch on and the current will be only the diode leakage current. This means that if one varies the rail voltage to the circuit of LED and resistor R, the current will be far less when the LED diode is off than when it is on.

I do see, however, how a resistor through a linear current measuring device does meet the definition. Just not with the diode in series. OR does the measurement of voltage matter only across the resistor?_________________FPGA, dsPIC and Fatman Synth Stuff

Time flies like a banana.Fruit flies when you're having fun.BTW, Do these genes make my ass look fat?corruptio optimi pessima

Sorry, didn't mean to confuse you, I said that mostly tongue-in-cheek.

Wiki does list a resistor in series with a voltage as the "simplest non-ideal current source" in as much as a resistor of a given value with a given voltage drop will produce a specific current as given by ohms law, but I admit that's a bit of a stretch._________________My synth build blog: http://gndsynth.blogspot.com/

So now that we've discussed a technicality for over a page can we get back to acctually using this as a control element in circuits?_________________As a mad scientist I am ruled by the dictum of science: "I could be wrong about this but lets find out"

Since it uses a MOSFET as the control element, the H11F1 is rather easy to use as a controlling element in a circuit. Essentially, anywhere that a variable resistance is used (not just a rheostat wired pot, but any resistance that varies) this device could be used. Care should be taken to note the MOSFET's resistance range and some modification of the original circuit may be required to accomodate that. It may also take some patience to work with the device's LED current to MOSFET drain-source resistance transfer function._________________FPGA, dsPIC and Fatman Synth Stuff

Time flies like a banana.Fruit flies when you're having fun.BTW, Do these genes make my ass look fat?corruptio optimi pessima

I'm going to send an email to one of the addresses in the whois info for his site;
http://www.whois.com/whois/whimsofdan.com
they are associated with his hosting company but maybe they can give me some leads which could get me in touch with him.

thanks cynosure _________________As a mad scientist I am ruled by the dictum of science: "I could be wrong about this but lets find out"

There are examples there for controlling a SVF, opamp gain and a variable resistor. Those should be a good start for whatever you are planning on doing._________________JacobWatters.com
Synth & DJ Product Specialist, Roland Canada

P.S. Turns out that dan is really fucking hard to find, all my leads are dead ends! His name may acctually be Dan Whims... I'll search some more._________________As a mad scientist I am ruled by the dictum of science: "I could be wrong about this but lets find out"

That is how I usually find most of my answers. Either that or Encyclopedia Joviannica

JingleJoe wrote:

P.S. Turns out that dan is really fucking hard to find, all my leads are dead ends! His name may acctually be Dan Whims... I'll search some more.

His YouTube name is 'dan a', so I assumed his last name started with an A.

He mentions in the video that he heard about the vactrols on the synth diy newsletter list, so you might want to ask there. You might get a response from him, but even if you don't, there was obviously some discussion there about these things. So there are likely more people there who have experimented with them.

As you posted above, his website was "whims of dan" (whimsofdan.com) and I've found a Dan Whims with a background in electronics and computers and music, if it's not the guy I'll be damned! _________________As a mad scientist I am ruled by the dictum of science: "I could be wrong about this but lets find out"

As you can see, there are a lot of intelligent and experienced people on that list. Plus, the person says in the YouTube video that he is on the list too._________________JacobWatters.com
Synth & DJ Product Specialist, Roland Canada

He actually uses the diodes in the circuit as a ladder, and then sends the transferred output to a separate transistor ladder._________________JacobWatters.com
Synth & DJ Product Specialist, Roland Canada

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